Segmental aortic hypoplasia or how to interpret the flow concept

Abstract

The development of elastin lamellae in the media of elastic type arteries such as the aorta appears to be modulated by mechanical factors such as flow. On that basis, decreased aortic flow during fetal development is held responsible for the occurrence of tubular hypoplasia of the aortic arch. Underdevelopment of the ascending aorta in aortic atresia likewise is considered due to diminished retrograde flow from the aortic arch. The present study has examined this hypothesis by comparing the number of elastic lamellae in hypoplastic segments of the aortic arch and ascending aorta with corresponding segments from normal aorta. In both situations the underdeveloped segments show a paucity of cellular and supportive connective tissue constituents, leading to a densely packed layer of elastic lamellae and, thus, contributing to the diminished dimensions. A major difference, however, was noted with respect to the number of elastic lamellae. In the hypoplastic ascending aorta in aortic atresia, the number was not different from that encountered in normal specimens. In tubular hypoplasia, on the other hand, the number of elastic lamellae was significantly lower than in the corresponding segments of normal specimens ( P < 0.01). Taking the stance that flow is a major factor in proper development of the aortic media, it seems that the findings in tubular hypoplasia fit well with a chronic lack of tension from early development and, hence, relate directly to the cardiac malformation. The findings in the ascending aorta of aortic atresia are less easy to understand from this point of view. They either discard flow as a prime factor or indicate that sufficient blood has been propelled through the developing ascending aorta, prior to closure of the aortic orifice, to enable the elastin skeleton to develop almost normally. The present findings, therefore, should give further thought to factors involved in the early development of the elastic lamellar skeleton of the aorta and to the “flow hypothesis” as the denominator of underdevelopment of aortic segments.